The surface structure of a sample can be observed by scanning and irradiating the sample with an electron beam and detecting secondary charged particles emitted from the sample. This is called a “scanning electron microscope” (hereinafter referred to as a SEM). On the other hand, the surface structure of a sample can also be observed by scanning and irradiating the sample with an ion beam and detecting secondary charged particles emitted from the sample. This is called a “scanning ion microscope” (hereinafter, referred to a SIM). Particularly, the irradiation of the sample with ion species with light mass such as hydrogen, helium, or the like makes a sputtering function become relatively small, so that the irradiation is suitable for sample observation.
The ion beam has a characteristic of being sensitive to information on the surface of the sample in comparison to the electron beam. This is because the excited area of secondary charged particles is localized according to the surface of the sample in comparison to the irradiation with the electron beam. In addition, with respect to the electron beam, since the properties of electrons as a wave cannot be neglected, aberration occurs due to the diffraction effect. On the other hand, with respect to the ion beam, since the ion is heavier than the electron, the aberration due to the diffraction effect is very small in comparison with the electron beam.
If the electron beam is irradiated on the sample and the electrons that have transmitted through the sample are detected, information on the internal structure of the sample can be obtained. Similarly, if the ion beam is irradiated on the sample and the ions that have transmitted through the sample are detected, information on the internal structure of the sample can also be obtained. This is called a transmission ion microscope. Particularly, the irradiation of the sample with ion species with light mass such as hydrogen, helium, or the like makes a ratio of transmission through the sample become large, so that the irradiation is suitable for sample observation.
On the contrary, the irradiation of the sample with ion species with heavy mass such as oxygen, nitrogen, argon, krypton, xenon, gallium, indium, or the like is suitable for sample processing due to the sputtering function. Particularly, a focused ion beam apparatus (hereinafter, referred to as an FIB) using a liquid metal ion source (hereinafter, referred to as an LMIS) is known as an ion beam processing apparatus. In addition, a plasma ion source or a gas field ion source may generate gas ions of oxygen, nitrogen, argon, krypton, xenon, or the like and the gas ions are irradiated on the sample, so that the sample processing can be performed.
In an ion microscope mainly for the sample observation, the gas field ion source is very suitable as an ion source. In the gas field ion source, gas such as hydrogen or helium is supplied to a metal emitter tip having a curvature radius of about 100 nm at the distal end, and a high voltage of several kilo voltages or more is applied to the emitter tip, so that the gas molecules are field-ionized, and the ionized molecules are extracted as an ion beam. The ion source has a characteristic that, since an ion beam having a narrow energy width can be generated and a size of the ion source is small, a minute ion beam can be generated.
In the ion microscope, in order to observe the sample at a high signal-to-noise ratio, an ion beam having a high current density needs to be obtained on the sample. For this reason, an ion radiation angular current density of the gas field ion source needs to be high. In order to increase the ion radiation angular current density, the emitter tip may be cooled down to a very low temperature. PTL 1 discloses that a minute protrusion is formed at a distal end of an emitter tip so as to improve characteristics of an ion source. NPL 1 discloses that a minute protrusion at a distal end of an emitter tip is made of a second metal different from a material of the emitter tip. NPL 2 discloses a scanning ion microscope equipped with a gas field ion source emitting helium ions.
PTL 2 discloses a structure which is configured to include an emitter for a charged particle beam is provided to first and second supporting portions, a filament extending therebetween, an emitter distal end portion provided to the filament, and a stabilization element provided for a third supporting portion and the filament, wherein the first, second, third supporting portions defines a triangle, so that the stabilization element at least partly extends in a direction perpendicular to the extension direction of the filament. According to the structure, a vibration amplitude of the emitter distal end portion is suppressed, so that resolution of a charged particle beam apparatus employing the emitter can be improved.
PTL 3 disclose an electronic source including a needle-shaped tip having an electron emission portion at one end, a cup-shaped part jointed to the other end of the needle-shaped tip different from the one end thereof, and a filament for heating the cup-shaped part, wherein the filament is disposed in an air gap defined inside the cup-shaped part so as not to be connected to the cup-shaped part. Although external vibration is applied to an apparatus using the electronic source, the apparatus can emit a stabilized electron beam.
PTL 4 discloses a structure of an electronic source where a pair of conductive terminals are provided for an insulator, a tip having an electron radiation portion is bonded to a filament attached between the conductive terminals, and the other end of the tip different from the electron radiation portion is fixed to the insulator, a structure of an electronic source where the other end of the tip different from the electron radiation unit is fixed to the insulator through a metal pin soldered to the insulator; and a structure of an electronic source where a curved portion is provided to the filament. Although external vibration is applied to an apparatus using the electronic source, the apparatus can emit a stabilized electron beam.
PTL 5 discloses a structure of a gas field ion source where a filament is supported by a plurality of pillars, for example 3, 4, 5, or 6 pillars. Sensitivity of an emitter to mechanical vibration can be reduced.
In addition, PTL 6 discloses a thermal field emission electron gun which is characterized in that, in a thermal field emission cathode capable of radiating a stabilized electron beam effective to the purpose of a high speed electron beam exposure apparatus or the like, a filament for heating a needle-shaped electrode has a structure of a V shape, and an angle of the V shape is in a range of 30 degrees to 90 degrees.